Video cassette recorders are generally called VCRS, and the term, VCR, will be used hereinafter to identify a video cassette recorder. It is a common practise in Beta type VCRs to frequency modulate each of the left and right channel audio signals onto respective ones of a plurality of FM carriers which are subsequently recorded on the video magnetic tape via the same rotating heads that are used to record the associated FM video and chroma information.
There are two different heads which are used for this purpose, one being referred to hereinafter as head A and the other as head B. Only one of these two heads is in operative contact with the tape at any given time. In a conventional Beta type VCR there four different FM audio carriers routed to the heads A and B. These carriers are respectively at frequencies of 1.38 MHz, 1.53 MHz, 1.68 MHz, and 1.83 MHz. The 1.38 MHz and the 1.53 MHz carriers are frequency modulated by the left channel audio signal while the 1.68 MHz and the 1.83 MHz carriers are frequency modulated by the right channel audio signal. The FM modulated 1.38 MHz and the 1.68 MHz carriers are connected to the head A and the 1.53 MHz and 1.83 MHz carriers are connected to the head B.
One method which is currently being used to troubleshoot the stereo audio circuits of a Beta type VCR is simply to analyze the signals at different places in the audio circuits with the aid of an oscilloscope. While signal analysis works well when examining the demodulated audio signals after the demodulators inasmuch as these audio signals are relatively distinct and uncomplicated. However, the audio modulated FM carriers are complex and difficult to analyze. In addition, these modulated signals are switched at a frequency of 30 Hz, making it still more difficult to analyze the modulated signal by simply observing it on an oscilloscope. For example, if a circuit has an input but no output, it is apparent that the circuit is defective. If, on the other hand, there is an output but it is incorrect yet beyond the technician's ability to determine whether is it right or wrong when simply using an oscilloscope, the technician may assume that the circuit is working correctly when in fact it is operating incorrectly.
Moreover, Beta type VCRs include a signal detector circuit which mutes the audio signal in the absence of an output signal from any of the audio circuits thereby to prevent improper audio signals from being passed to any of the output amplifiers and from them to the speakers. This muting function makes analyzing of the stereo audio circuits virtually impossible when the audio circuits are muted and there is no audio output to observe.